Banknote security features

As long as there have been banknotes, there have been people trying to copy them. The Bank of England started issuing notes in 1694, with Daniel Perrismore becoming the first person to be arrested for forgery in 1695. Forgery isn't too popular with those who issue banknotes and there is a constant race between those trying to create security features that cannot be copied and those trying to copy them. Some of these features are aimed at professional gangs with very good equipment, whereas others are aimed at the more casual forger who have become increasingly common with the advent of comparatively cheap colour scanners and laser printers (a few years ago a French teenager used his school's laser printer to produce fake banknotes). In some cases, one is even up against other countries, it is though that some countries of the Middle East (in particular Iran and Syria) have been producing high quality counterfeit US dollars for several decades, both for their economic advantage, and to weaken the dollar.

Your average banknote is printed using sophisticated equipment, and uses a combination of techniques to make the work of forgers harder. The Bank of England uses a combination of offset printing, Intaglio printing and letterpress printing. Many notes include some of the following processes:

Raised print:
Some parts of the note may be printed with thicker ink in such a way that can be felt if one runs a finger along it. This can also be used to make notes more easily identifiable by the blind.

Micro-print:
Micro-print is a series of characters printed extremely small (almost unreadable with the naked eye). Inferior equipment will not be able to scan or print such fine detail and will only render solid lines. A related security feature is a series of fine lines, printed very close together. The size of these features is chosen so that commercially available printers or copiers will have insufficient resolution to reproduce them accurately.

"See through" features:
Half of some number, text or graphic is printed on either side of the note. If one lifts the note to the light, the 2 halfs should fit together perfectly, like 2 pieces of a jigsaw puzzle. If you were to try and copy such a feature it is likely that when printing the second side there would be a slight shift.

Moiré patterns: These patterns that are created by the interference of 2 separate patterns tend to disintegrate when copied.

In addition to these specific features, banknotes tend to have increasingly complex designs, full of swirling and overlapping patterns that are hard to reproduce.

It doesn't take a genius to notice that banknotes aren't printed on cheap copier paper. Even without considering security, banknotes need to be able to withstand being folded and crumpled, stuffed into pockets and purses over and over again. Most countries use a special paper with cotton fibres and/or linen that give banknotes their distinctive feel (and of course there are the new polymer notes used in countries such as Australia or New Zealand). As an extra security feature some fibres may be treated so that they become fluorescent when exposed to ultraviolet light.

Polymer notes!

The polymer based notes used in increasing number of countries offer many new possibilities. They can contain transparent windows that filter out a specific colour making a hidden pattern visible or that act as a lens, enabling one to see the micro-print on the note. These "self authenticating" features are critical as it is important that anyone can quickly and easily check the authenticity of a banknote.

The polymer substrate itself is more difficult to copy. It was not until several years after the introduction of polymer notes in Australia (who pioneered the technology) that the first polymer forgeries were found there (although there had been paper forgeries, however they were easily detected as they felt very different from the polymer notes) and they have so far been of poor quality (with the transparent window being a particularly hard to copy feature). The polymer notes also have the advantage that they are tougher and easier for machines to manipulate.

Embedded objects

Many banknotes now include objects embedded or integrated into the paper, these include:

Watermarks: One of the older security features (used by the Bank of England as early as 1697), watermarks are barely visible normally, but when held up to the light, some parts appear darker, thus creating a picture.

Security thread: A thread is embedded in the paper. It cannot be seen by simply looking at the note, however if one holds the note up to the light, it becomes visible as a dark line across the note. Sometimes this thread contains information that can be read by machines. On euro notes, the thread contains a code that authenticates the banknote and indicates its value.

Strap: a strip of foil is embedded in the paper, in such a way that only part of it is visible on each side (think dotted line).

RFID tags: the ECB is apparently planning on including these in its banknotes as early as 2005, although it refuses to comment. There is however talk that the ECB has been negociating with Hitachi, who make the world's smallest RFID chip (0.4mm square, 0.1mm thick). There are rumors that the system is already in place in euro and possibly other banknotes, however this seems unlikely as manufacturing processes are not yet really up to the challenge of a banknote, which is routinely folded, unfolded accidentally put through the wash etc. The alleged purpose is to combat fraud and money laundering, although many will no doubt sense privacy issues.

Foil embossing

As their name indicates, these are silvery strips of foil that are incorporated into the paper's surface. When photocopied they appear black. As an added security measure, one can use holographic strips or marks,
These usually depict a national emblem or the value of the note. When one tilts the note the colours change and sometimes certain features of the hologram may only be visible under certain angles.

It is obviously far easier to copy a banknote of one single colour than it is to copy one using subtle gradients and colours that are traditionally hard to reproduce accurately (often pale greens, blues or yellows). Banks try and make the job even harder by using special inks whose colours are hard to reproduce with commonly available inks. The Bank of England uses 85 different inks to produce its 4 denominations.

It is common to use inks that are only visible under ultraviolet light, or metallic "colour shifting" inks that change colour depending on the angle of viewing. Iridescent strips that are transparent when viewed straight on but which shift colour when viewed from the side are also used. Metameric inks (special inks that change colour when viewed through a certain type of colour filter) are used to create messages that cannot be seen without one of the aforementioned filters.

Digital Craftiness

For all the sophisticated methods used, a good colour photocopy, or a scan with a bit of Photoshop can go a long way in a busy shop where the shopkeeper is too rushed to check. To help combat this, if you look on certain modern banknotes, such as euros, pounds or the new American $20 bill you can spot little circles in a pattern that looks a little like this :

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The circles are around 1mm in diameter on the euro notes I have here, and are pale green or yellow, although they are much more visible when an appropriate filter has been applied. It is easy for a piece of software to test for the presence of such a configuration, and that is exactly what the firmware of high-end photocopiers does. Recent versions of Photoshop also implement such a test and will refuse to open files representing certain currencies.
For some pictures of banknotes with these circles, go to http://www.cl.cam.ac.uk/~mgk25/eurion.pdf.

Banknotes have certainly come a long way from their debut. Early Bank of England notes were black and white printed on only one side. But despite the sophistication of all of these copy protection systems in use today, it is a simple fact that none are infallible, what one man can make another can usually copy. However the combination of numerous different features makes the job much harder for forgers as they need to master an ever increasing range of techniques, and put it out of the reach of most organisations.